The flight management computers installed on the present generation of commercial jet aircraft provide a new technique for reaching a defined end-of-descent point with great precision. The technique involves construction of a descent path, based on the idle thrust performance of the aircraft, which descent path begins at a computed top-of-descent position and terminates at the desired end-of-descent position.
Ideally an aircraft, once cleared for descent, is left alone to perform that descent without further interaction with the Air Traffic Control (ATC) system. In the current ATC environment, however, descent clearances are to some intermediate altitude with progressively lower altitudes assigned as the descent proceeds. Whenever a lower altitude clearance is not received in a timely manner, the crew is required to level the airplane at the last altitude to which they have been cleared. When leveling occurs, the airplane travels above the desired idle thrust descent path.
In prior flight control systems, when leveling in descent, the throttle comes forward (increases thrust) to maintain the descent target speed. With respect to the optimum idle descent path, this throttle activity represents unwanted energy being added to the airplane. Once the aircraft is cleared to resume the descent, speed brakes must be used as the airplane regains the descent path to dissipate the energy added by the increased thrust during the level flight segment. The addition and dissipation of this energy results in an increased fuel cost for the flight. In addition, as the aircraft descends to regain the desired idle thrust descent path, the crew has no direct cue indicating when sufficient energy has been dissipated by the speed brakes to enable the aircraft to regain the desired descent path without further use of the speed brakes or throttle.